The present invention relates to a process for making a zirconia toughened alumina having an extremely small particle size, high density, high flexural strength and fracture toughness in a low-cost, simple manner.
Zirconia toughened alumina (ZTA) is a high strength ceramic used in a variety of extreme environmental conditions. The ceramic of zirconia in a continuous matrix of alumina has generally been found to have superior fracture toughness, but at the expense of mechanical strength. The ability to obtain high toughness while maintaining suitable strength has been a major concern of the art.
It has been proposed that both said fracture toughness and flexural strength can be attained by providing the continuous ceramic matrix with extremely fine microfissures in U.S. Pat. No. 4,298,385. In that patent, ceramic powders are obtained by wet milling the ceramic oxide to the appropriate particle size. The zirconia is of a 2 to 15 micron average agglomerate size, whereas the alumina is ground to the submicron range. The microfissures in the matrix are created by the interdispersed zirconia particles undergoing a phase transition (and concomitant volume expansion) from the metastable tetragonal crystalline form to a stable form upon cooling from sintering to room temperature. However, this increase in fracture toughness from the microfissures causes a decrease in flexural strength. This process also relies on time and energy-intensive grinding to obtain the desired particle sizes.
The use of interdispersed, fine (submicron range) grain zirconia in an alumina matrix has also been proposed in various other patents. U.S. Pat. No. 4,218,253 discloses the use of metastable tetragonal zirconia particles in the submicron size range interdispersed in an alumina matrix. It is alleged that these small particles give a ceramic body with both high strength and fracture toughness without the need for microfissures. U.S. Pat. No. 4,316,964 is similar to U.S. Pat. No. 4,218,253 and discloses the addition of various stabilizing agents such as yttria so that larger sized grains of zirconia can be used in an alumina matrix without transformation into the stable monoclinic form. The object in both of these patents is to have zirconia in the metastable tetragonal form at room temperature on the theory that when a crack forms, the metastable tetragonal ZrO.sub.2 transforms into the stable monoclinic structure at the crack front thereby increasing the overall fracture toughness.
U.S Pat. No. 4,314,827 discloses the use of a zirconia gel which was dried and coarsely ground approximately to 150 microns) in a ceramic (e.g. alumina and magnesia) matrix with an acceptable density for use as an abrasive. The zirconia gel was formed from zirconyl alkanate and peptized alumina monohydrate.
Production of high density ceramic grade ZTA particulate powders is disclosed in U.S. Pat. No. 4,532,224. The process of the patent involves a flash evaporation in a specifically designed high temperature flash furnace to obtain extremely fine zirconia particles which are naturally stable in the tetragonal form (less than 0.2 .mu.m). The extremely fine grained particles are centrifuged and dried to remove the water used in the rapid quench after the flash evaporation. There is no discussion of the agglomerate size, however. The process is complex, requires fine scale turbulent atomization, and the product cannot be handled easily.
The use of coprecipitation as a method of preparing zirconia ceramics was proposed by Japanese Patent No. 54-25523. A precursor zirconia salt of a nitrate or an oxychloride with additive amounts of stabilizer salts or oxides is co-precipitated with aqueous ammonia. The thus precipitated powders are made anhydrous by azeotropic distillation using added amounts of an organic solvent. Residual organic solvent is removed by drying at atmospheric pressure at 150.degree. C.
U.S. Pat. No. 4,501,818 discloses coprecipitating zirconia and yttria in anhydrous ethanol with sodium hydroxide. The precipitate is dried, washed with water, and dried again. The object was to avoid filtering problems and yield a sinterable powder. However, the repetitive drying and need for anhydrous ethanol makes the process unacceptable for most industrial applications.
Despite the above efforts, a simple straightforward method of obtaining sinterable ZTA powders which have homogeneously dispersed fine particulates, a high density, high flexural strength and fracture toughness is still sought by the art.